1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image in, for example, electrophotography, electrostatic recording, and electrostatic printing, and relates to a developer, a toner container, a process cartridge, an image forming method, and an image forming apparatus.
2. Description of the Related Art
In recent years, market demands for energy saving and higher-speed performance in image forming apparatuses such as printers, copiers, and facsimiles have been stronger. In keeping with this trend, in the field of electrophotographic toner (hereinbelow, otherwise referred to as “toner” for simplification) as well, while a toner excellent in low-temperature fixability is required, a toner having properties that work against low-temperature fixability, for example, offset resistance, heat resistant-storage stability (blocking resistance) and smear resistance on developer rollers etc., is also required.
To respond to these demands, the following toners have been proposed. Specifically, a toner which contains a linear polyester resin having defined physical properties such as molecular weight (for example, see Japanese Patent (JP-B) No. 3773906), a toner containing a non-linear crosslinked polyester resin in which rosins are used as an acid component (for example, see Japanese Patent Application Laid-Open (JP-A) No. 04-70765), a toner having improved fixability by using a rosin modified with a maleic acid (for example, see Japanese Patent Application Publication (JP-B) No. 07-82254), and a toner using, as a binder resin, a polyester composed of an alcohol component and a carboxylic acid component containing a (meth)acrylic acid-modified rosin (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 2007-292860 and 2007-292869). Also, there has been proposed a method of blending a low-molecular-weight resin and a high-molecular weight resin (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2-82267).
With the recent market growth in the print-on-demand (POD) field, demands for toner have been increasing in the printing market. The POD technology utilizing an electrophotographic printing method is well suited for printing a small number of copies and for variable printing (printing of images or data varied for each paper sheet) and thus is expected as an alternative to low-volume printing technology. However, as there is a need for a super high-speed printing system that operates at a significantly faster printing speed than the conventional high-speed copiers, and compatibility with a wide variety of paper sheet types, there is new need for a toner capable of exhibiting excellent fixability even with a smaller amount of heat and causing less smear on developing rollers and the like.
As described above, in the field of print-on-demand (POD) using electrophotographic printing methods, there is need for a super high-speed printing system that operates at a significantly faster printing speed than the conventional high-speed copiers and that is compatible with a wide variety of paper sheet types. Therefore, the toner consumption rate is high, and it is undesirable to use a toner which is inferior in pulverizability and productivity, like toners containing a linear polyester resin having defined physical properties such as molecular weight, as seen in JP-B No. 3773906. Also, rosins used in JP-A No. 04-70765 and JP-B No. 07-82254 are effective in improving the low-temperature fixability, but have a drawback in that these toners are liable to generate odors depending on the type of rosin. Furthermore, toners disclosed in (JP-A) Nos. 2007-292860 and 2007-292869 use as a binder resin a polyester composed of an alcohol component and a carboxylic acid component containing a (meth)acrylic acid-modified rosin. These toners can exhibit excellent fixability on a wide variety of conventional type image forming apparatuses ranging from low-speed printing machines to high-speed printing machines. However, these toners fail to simultaneously satisfy the low-temperature fixability and smear resistance on a carrier and developing rollers and the like on super high-speed printing systems, and still remain inadequate to meet the above-mentioned demands in the print on demand (POD) field.
Conventionally, a so-called “pulverized toner” is widely used as a dry-process toner in, for example, electrophotography, electrostatic recording and electrostatic printing. Pulverized toner is a toner in which a toner binder (e.g. a styrene resin and a polyester resin) is melt-kneaded together with a colorant, and the melt-kneaded product is pulverized. Such a pulverized toner requires not only a large amount of energy consumption for melt-kneading and pulverization, but also requires a step of classifying pulverized toner particles so as not to widen its post-pulverization particle size distribution, further leading to a decrease in productivity. Amid demands for improving the low-temperature fixability of toner to respond to the recent demands for higher-speed performance and energy saving, the softening point of a resin is decreased and toner particles are excessively pulverized in pulverization processes, which leads to a new problem of low productivity of pulverized toner.
A pulverized toner is produced by melt-kneading and dispersing internal additives such as a colorant, a charge controlling agent and a releasing agent in a binder resin (for example, see Japanese Patent (JP-B) Nos. 2851895, and 3772910). However, in such a pulverization method, particles tend to be pulverized at the interface between the internal additives and the binder resin during pulverization process, causing nonuniformity between individual toner particles and nonuniformity over the surface of individual toner particles, which leads to degradation in the quality of toner. Even if such toner particles are subjected to classification, image quality may vary after repeating the developing process, because of the wide particle size distribution of the toner. This is because particles having particle diameters which are easily developed in the developing process exist in the toner particles. In a two-component developer using a carrier, toner particles having large particle diameters tend to be used in developing processes. Therefore, when a developing process is repeated, the diameter of toner in the developer is reduced to cause degradation in image density. Meanwhile, in a one-component developer, toner particles having small particle diameters are easily developed, and when a developing process is repeated, the particle diameter of toner in the developer becomes greater, which tends to cause degradation of the dot reproducibility and the middle toner reproducibility.
With solutions to the drawbacks associated with pulverized toners, recent interest has focused on a so-called polymerization toner produced by toner production methods such as suspension polymerization methods, emulsion polymerization aggregation methods, and polymer dissolution/suspension polymerization methods. In a toner produced by a polymerization method selected from the suspension polymerization method, emulsification polymerization method, and dissolution suspension method, a styrene-acrylic ester copolymer is generally used. As a toner using a polyester resin, there has been proposed to use a polyester modified with a urea bond for the purpose of improving the heat resistant storage stability and the low-temperature fixability (for example, see Japanese Patent (JP-B) No. 3762077), and to use a polyester which is produced by polycondensation of 1,2-propane diol with a carboxylic acid component containing a purified rosin (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2008-281882). Concerning the use of polyester in toner, in the recent super high-speed printing systems using an electrophotographic printing method in the print-on-demand (POD) field, it is also desired to achieve improvements in the low-temperature fixability, and the offset resistance, heat resistant storage stability (blocking resistance) and smear resistance on developing rollers, which are properties opposite to the low-temperature fixability and to reduce the amount of odor that could be caused depending on the type of rosins.